Positive grip lifting clamp with direct hand lock



Sept. 16, 1958 E. M. GARDNER 2,852,300

POSITIVE GRIP LIFTING CLAMP WITH DIRECT HAND LOCK.

Filed April '1, 1954 2 SheetsSheet 1 'IIIIII I fi l/[IA INVENTOR. 1Edward Merrill Gardner ATTORNEYS Sept. 16, 1958 E. M. GARDNER 2,852,300

POSITIVE GRIP LIFTING CLAMP WITH DIRECT HAND LOCK Filed April 7, 1954 2Sheets-Sheet 2 as 52' 53 52' s2 a? as 9 F|G.7 w r 7 INVENTOR.

Edward Merrill Gardner BY a 7 M ZQL A M ZWYJZ /Z'z.

ATTORNEYS United States Patent POSITIVE GRIP LIFTING CLAMP WITH DIRECTHAND LOCK Edward Merrill Gardner,

Merrill Brothers, New York, N. New York Application April 7, 1954,Serial No. 421,647 7 Claims. (Cl. 294104) Y., a corporation of such aclamp having a directly operating hand-operated locking means andadapted to grip and lift hard steel plates.

The lifting clamp of the present invention is an improvement on thelifting clamps disclosed in my Patent No. 2,393,101, granted January 15,1946, and my pending application Serial No. 278,864, filed March 27,1952, now Patent No. 2,676,838, granted April 27, 1954.

In my pending application I have disclosed a plate lifting clamp whichis adapted to effect a substantially tight grip on ordinary steel plateswhen the lift or pull is vertical, horizontal or at any angle inbetween. However, it has been found that when it is necessary to lifthard steel plates, such as heat treated armor plates or alloy orstainless steel plates which have a smooth surface, it is difficult tomake the gripping cam of the lifting clamp penetrate the sheet to belifted sufficiently to get a good grip, especially at the start of thelifting operation. In this connection it has been found that, if the camteeth do not penetrate the plate somewhat, it is impossible to get theclamp to function, since no load is applied on the lifting shackle ofthe clamp. Furthermore, it has been found that the penetrating forcelifting operation, and upon a small contacting surface in order to causethe cam teeth to penetrate the plate.

The primary object, therefore, of the present invention is to provide animproved lifting clamp which includes a locking mechanism for positivelyforcing and holding the cam of the lifting clamp in locking engagementwith the plate to be lifted.

A further object of the invention is to provide a lifting clamp for hardsteel plates which includes means for forcing the teeth of the grippingcam into the plate to be lifted, thereby avoiding damage and accidentscaused by the clamp having slipped off the plate.

According to the invention, the improved lifting clamp includes astructure generally similar to that disclosed in my pending applicationand provided with hand-operable means acting directly on the grippingcam of the clamp for forcing its teeth into the surface of the plate tobe lifted, thereby providing a positive grip.

According to a preferred construction, the improved clamp incorporates anovel locking mechanism in which the back portion of the gripping cam tobe clamped is formed with a cam surface having a predetermined pitch orgrade to provide one element of the locking mechanism, the other elementof the mechanism including a locking cam device having a cam surfaceadapted to act upon the back cam surface of the gripping cam to forcesome of the teeth of the gripping cam into the surface of the plate tobe lifted. In this construction the cam surfaces of the locking-andgripping cams are so shaped relative to each other that a positive lockis applied to the gripping cam which cannot be released by any pres-Brooklyn, N. Y., assignor to i must be great enough at the start of the2,852,300 Patented Sept. 16, 1958 sure applied backwardly to the plategripping cam. According to this preferred construction, the locking orauxiliary cam is provided with a grade or pitch which is about one-halfthat of the back cam surface of the gripping cam, and the point oftangency and engagement of the cam surfaces is located inside of astraight line between the centers of rotation of the cams.

In a modified form of clamp construction, the means for positivelyforcing the gripping cam against the plate to be lifted and locking itin that position comprises a special lug on the back portion of thegripping cam and a cooperating readily releasable screw in line with theback of the cam adapted to engage the lug on the gripping cam and forceits gripping surface into contact with the plate to be lifted.

The invention includes other objects, features and advantages, which aredescribed more in detail hereinafter in connection with the twoembodiments of the lifting clamp, as shown in the accompanying drawingsforming a part of this application.

In the drawings:

Fig. 1 is an elevational view partly in section of a lifting clamp indisengaged position embodying the features. of the present inventionshown with one of the side plates of the clamp removed;

Figs. 2 and 3 are views similar to that of Fig. 1, respectively showingthe position of the gripping cam and other. parts as they are beingmoved into locking position, and in locking position, respectively;

Fig. 4 is an enlarged detailed view, partly in section, of the lockingscrew mechanism comprising a part of the lifting clamp shown in Figs. 1to 3;

Figs. 5 and 6 are views similar to those of Figs. 1 and 3 illustratingthe preferred embodiment of the invention in which a locking orauxiliary cam is used in cooperation with the gripping cam of thelifting clamp, Fig. 5 showing the gripping cam in disengaged position,while Fig. 6 shows the gripping cam in engaged position locked onto aplate to be lifted; p

Fig. 7 is a vertical sectional view of the lifting clamp shown in Figs.5 and 6, taken on the line 7-7 of Fig. 5;

Fig. 8 is a broken side elevational view of the lifting clamp shown inFigs. 5 to 7 showing the position of the hand-operated lever when thecams are in disengaged position; and

Fig. 9 is a broken sectional view taken on the line 99 of Fig. 8.

Referring to Figs. 1, 2 and 3 of the drawings, the improved platelifting clamp shown therein comprises a clamp body It having a short legor jaw 12 and a long leg or jaw 14, spaced and facing each other toprovide a slot 16 for receiving a plate 18 to be gripped and lifted bythe clamp. The short jaw 12 is provided with a serrated gripping pad 20extending slightly into the slot 16 and facing the longer leg or jaw 14.I

The body 10 of the clamp includes a pair of spaced side plates 22 and24, which are held in spaced relation by spacing blocks 26, 28 and 30and riveted together by means of rivets 32. The spacing blocks 26 and 28may have a thickness equal to the space between the plates or. may havea thickness of only half the distance between the plates, the separatehalves being integral respectively with the plates 22 and 24, so thatthey engage each other I when the plates are riveted together. Thespacer 30 serves as a stop for the gripping cam of the clamp.

The longer jaw 14 includes and houses a gripping cam- 34, the lowerportion of which is pivoted on a pivot pin 36 located in the lowerportion of the jaw 14. The cam 34 includes an enlarged hubportionmounted between the side plates 22and 24, while the pin 36 is of thecotter pin type held in the body of the clamp by a cotter pin, notshown.

The 'leverage mechanism for actuating the serrated cam 34 into and awayfrom gripping engagement with the plate 18 includes a radius link 38mounted between the side plates 22 and 24 in the upper portion of theclamp body and pivoted therein on a pivot pin 40 held in place by acotter pin, not shown. The outer arm 42 of the radius link is providedwith an end portion having the same thickness as the cam 34, this armbeing connected to the cam by a pair of connecting links 44,respectively arranged on opposite sides of the arm 42 and gripping cam34 and being pivoted thereto by pivot pins 46 and 48, respectively, theedges of each of which may be peened over into a counterbore to holdthem against axial movement.

The other arm 50 of the radius link 38 is pivoted to the inner end of aforked connecting-lifting shackle 52 by means of a pin 54 held in placein the same way as pin 46. The lifting-connecting shackle 52 may includea lifting ring adapted to receive a hook normally carried by a liftingcrane or other lifting apparatus.

Most, if not all, parts of the clamp are of heavy forged steel and theleverage and lifting elements of the clamp are arranged so that theyreadily fit within the side walls 22 and 24. For example, the cam 34 hasa relatively large hub which fits between the plates 22 and 24, whilethe web of the cam is engaged on opposite sides between links 44, whichalso engage the opposite sides of the radius link arm 42. The radiuslink 38 includes a thick hub section integral with its ends which fitsbetween the side plates 22 and 24, while the forked end of the liftingshackle 52 fits between the side plates 22 and 24, as they span the end50.

According to the modified form of the invention shown in Figs. 1 to 4 ofthe drawings, the means for positively locking the clamp onto the plateor other object to be lifted comprises a screw 56 to be applied againsta projecting lug 58 provided on the back of the cam 34. The lug 58 isthe same thickness as the cam proper and is provided with a back contactsurface arranged at approximately 90 to the axis of the screw 56 whenthe cam is in a number of different gripping positions. The shaping ofthe back of the lug in this way avoids any binding action with the screwand the bending of the screw when pressure is applied.

The screw 56 is advantageously an Allen type screw having a somewhattapered and rounded point for engaging the lug 58 and is mounted in abore 59 of a housing 60 located at the back of the clamp. The housing 60may be welded to the clamp plates 22 and 24 or be integral with theseplates in such a manner that it comprises a sturdy block-like structurelocated with the bore for the screw 56 somewhat above the pivot pin 36for the cam 34, so that the screw will be able to app-1y a considerableleverage pressure against the cam and rotate it clockwise toward theplate to be engaged. The bore 59 in the housing 60 is smooth and extendsat right angles to the slot 16 of the clamp and in alignment with thecam 34.

A movable wedge-shaped quick releasing semi-nut 62 is mounted in thelower portion of the housing 60, and its upper surface is provided withthreads engageable by the threads of the screw 56. The semi-nut 62 is ablock-like structure slidable toward and away from the screw 56 in apassageway 64 inc-lined with respect to the axis of the screw. Acompression spring 66, the ends of which are respectively engaged inholes in the housing 60 and semi-nut 62 biases the semi-nut toward itsupward position where its threads engage the threads of the screw 56.

The semi-nut 62 is retractable manually by means of buttons 68 (Fig. 4)arranged on opposite sides of the housing 60 and respectively carried onthe ends of a pin 70 extending through slots 72 in the opposite sides ofthe housing, and through the body of the semi-nut 62. A guard 74 toprotect the screw and housing from damage is advantageously secured tothe body of the clamp and extends around the sides and upper part of thehousing 60 and screw 56 in spaced relation to the housing so that anoperator may readily actuate the buttons 63 to release the screw 56.

In the operation of the clamp shown in Figs. 1 to 4- for gripping theplate 18, the operator first presses the buttons 68 toward the body ofthe clamp against the action of the spring 66 to disengage the nut 62from the screw 56, as in Fig. 2. This is accomplished by one hand, whilethe operator with the other hand presses the clamp shackle 52 toward thebody of the clamp causing the cam to pivot counterclockwise and push thescrew 56 back into the position shown in Fig. l of the drawings. Nowwhen the slot 16 of the clamp extends over the plate 18, the buttons 68are released and the semi-nut 62 engages the screw 56, as in Fig. 1. Theshackle 52 is now pulled out in the usual manner in the direction of thearrow causing the cam 34 to move away from the screw 56 and engage theplate 18, as in Fig. 2. At this point the screw 56 is pressed inwardlyby hand until its inner end engages the back of the cam 34. As the screw56 is pushed inwardly toward the cam 34, and during its inward movement,since its thread is engaged with the thread of the semi-nut 62, itforces the semi-nut 62 to travel along the passageway 64, and becausethe bore 59 and passageway 64 diverge, the seminut 62 and the screw 56separate, or their threads disengage until the top of the thread of thescrew 56 passes over the thread of the semi-nut. As the screw 56 ispushed inwardly to the position shown in Fig. 2, its point engages thelug 58 at the back of the cam 34 and, as soon as the forward movement ofthe screw 56 is arrested, the spring 66 slips the semi-nut 62 up alongthe passageway 64 to engage its thread with the thread of the screw 56.An Allen wrench, which may be clipped to the body of the clamp forconvenience, is then applied to the Allen screw 56 forcing the screwagainst the lug 58, which in turn forces some of the teeth of the cam 34into the plate 18, before the plate is lifted. The sharp edges on theface of the gripping pad also enter the surface of the plate to someextent. Fig. 3 shows the clamp in this position with the teeth of thecam 34 pressed into the plate.

With the clamp firmly gripped onto the plate, it may be safely lifted tothe desired point and laid down again. In order to release the clampfrom the plate, it is necessary to again use the Allen wrench to givethe screw 56 about 1%. to 2 turns backwards to release the tension andalso to allow play between the threads of the screw and nut. Theoperator then presses inwardly and downwardly on the buttons 68 tocompletely release the screw which is readily pushed back by the camwhen an inward pressure is applied to the shackle 52. In this operationit is not necessary to rotate the screw except for the first 1% to 2revolutions sufiicient to release the seminut 62.

The preferred form of locking clamp according to the invention is shownin Figs. 5 to 9 in which the body of the clamp is of the same generalform as that shown in Figs. 1 to 4 and in which like parts have beengiven the same numbers followed by a prime. As shown in Figs. 5 and 6, abell crank 39 is used in place of the radius link 38 with its outer arm43 pivoted to the links 44'. The bell crank 39 is arranged with itsinside angle facing generally toward the cam which does not include alug 58 as in Fig. 1 but is provided with a back cam surface used incooperation with the locking mechanism shown in Figs. 5 to 9.

The inner arm of the bell crank 39 extends into the forked end of alifting link 53 and is pivoted thereto by the pivot pin 54'. The outerend of the link 53 in turn extends into the fork of a lifting shackle52' and is pivoted thereto by a pivot pin 55, which may be held inplace'in the same manner as the pivot pins 46, 48 and 54. The use of thebell crank 39 in place of a straight radius link, such as 38, provides aconstruction in which the angle of pull by the lifting shackle 52 may beapplied in various directions without acting on the leverage mechanismto in any way tend to release the cam 35 from its grip on the plate 18'.Assuming, for

body plates 22' and 24. The cam 75 is fixed against lateral displacementon the pin or shaft 76 by a pin 83.

example, that the pull on the shackle 52' is at an angle of 90 to theright with respect to the slot 16', the arm 45 of the bell crank is sofar into the body of the clamp that there would be no possibility of theclamp being released from the plate 18'.

According to the preferred construction shown in Figs. to 9, theimproved locking mechanism for the clamp comprises an auxiliary cam 75mounted in the back portion of the body of the clamp and keyed to a pin76 in alignment with and somewhat above and in back of the major portionof a gripping cam 35. The cams 35 and 75 are respectively involuted camsor are provided with involuted engaging edge cam surfaces 77 and 78. Theinvoluted surface 78 of the cam 75 has a grade or pitch which is aboutone-half that of the involuted surface 77 of the gripping cam 35. Theterm grade or pitch means the angle in excess of ninety degrees which isformed by two intersecting lines, one through the center of rotation ofthe cam at the point where the other is tangent to the surface of thecam. This arrangement of the locking cam 75 is such that, when it isrotated to firmly engage the surface 77, a positive lock on the cam 35is provided, which is not dependent in any way upon the action of aspring or upon the holding of the auxiliary cam by hand. After the cam75 is moved into locking engagement with the cam 35 as in Fig. 6 it cannot be released by a backward pressure on the cam 35 i even though notheld by a spring or by hand. The secret of this locking action is therelative pitches of the two involuted cam surfaces 77 and 78 or the factthat in the relationship shown in Fig. 6 the sum of the distancesrepresented by the lines 92 and 93, respectively extending from thecenters of rotation of the cams 75 and 35, to the point of commontangency and contact 90 of the cam surfaces, is greater than thedistance represented by the straight line 91 extending between saidcenters of rotation.

On rotating cam 75 clockwise, because of the fact that the distance frompoint 98 to the center of rotation of cam 75 is longer than the radialdistance at any point on the cam surface 78 to the right of that line, aspace for disengaging of the two cam surfaces will occur and thereforeallow cam 35 to rotate counter-clockwise. In attempting to move cam 35counter-clockwise without first moving cam 75 clockwise when it isengaged with cam 75, an infinitesimal action is started in spite of thefact that the sum of the two radial distances 92 and 93 to point 90 islonger than a direct line between the center points of rotation. This isdue to slight strains of material under stress. However, the rollingcannot continue except an infinitesimal amount because the sum of thetwo radial distances when cam 75 rolls counterclockwise progressinglyincreases, causing increasing strains and stresses until there is abalance of forces and equilibrium results. The reason the sum of the tworadial distances increases in this instance is because the pitch orgrade on cam 75 is less than on cam 35.

The auxiliary cam 75 is provided with a biasing spring 79 of the clockspring type arranged on one side surface of the cam and having one endfixed thereto by means of a pin 80, the other end of the spring beingfixed to the inside of the body of the clamp by means of a pin 81. Thespring 79 biases the cam in a counterclockwise direction so that whenreleased, it follows the rotation of the gripping cam 35 in a clockwisedirection and prevents the reverse rotation of the latter.

A handle 82 is provided on the outside of the body of the clamp forrotating the auxiliary cam 75, this handle being fixed to the pin 76,for example, by a key as shown in Fig. 9. The pin 76 extends through theThe handle is retained in place by a cotter pin, as shown in Figs. 7 and8. When the auxiliary cam 75 is in its unlocked or released position, asshown in Figs. 5 and 8, the handle 82 is in the position shown in Fig. 8and is latched to the body of the clamp by means of a pin 84 (Fig. 9)engaging in a hole 85 in the body plate 24. The pin 84 is carried bytheouter end of the arm and biased to engaging position by means of aspring 86 mounted in a casing 87 threaded into the arm 82. A knurledbutton 88 on the outer end of the pin 84 permits the ready release ofthe arm 82 so that it may be swung upwardly either by the action of thespring 79 or by the hand of the operator. When the pin 84 is not engagedin the hole 85, as in Figs. 8 and 9, the inner end of the pin rides onthe outer surface of the body plate 24 in the manner shown in Fig. 7.

The operation of the clamp shown in Figs. 5 to 9 during the attachmentof the clamp to a'plate follows a sequence similar to that describedabove in connection with Figs. 1 to 4, Fig. 5 showing the gripping cam35 in released position and cam 75 held in its disengaged position. Nowas the lifting shackle 52 is pulled up, the cam 35 is rotated in aclockwise direction to move its teeth into engagement with the plate18'. At the same time the handle 82 is released so that the cam 75rotates in a counterclockwise direction with the surface 78 followingthe surface 77. When the plate 18 is firmly held between the teeth ofthe cam 35 and the gripping pad 20, the operator pulls back anddownwardly on the handle 82 to apply pressure on the cam surface 77 bythe cam surface 78, thereby forcing the teeth of the cam 35 into thesurface of the plate 18'. At the same time the teeth on the pad 20 willengage somewhat into the surface of the plate 18' so that the clamp isfirmly locked onto the plate.

As the involuted surface 78 is moved against the involuted surface 77,the former slips slightly on the latter and creates a mechanicaladvantage or leverage for pressing the teeth of the gripping cam forwardagainst and into the surface of the plate 18. The cam 35 is lockedagainst the plate 18 by the locking mechanism because the point oftangency or of engagement 90 of the two cam surfaces is on the inside ofthe straight line 91 extending between the axes of rotation of the cams.

After the plate 18" has been moved to the desired location, the clamp isreleased by operating the handle 82 to rotate the auxiliary cam 75 in aclockwise direction. In this operation there must be some slippagebetween surfaces 78 and 77, and the surface 78 must be moved twiceasfast as the surface 77if the cam 35 is rotated counterclockwiseduringthe release of the cam 75. The only way the cam 35 can be releasedis to first rotate the auxiliary cam 75 clockwise since it is jammed andwedged in against the surface 77 when the plate 18' is ready to belifted, as shown in Fig. 6. If an attempt is made to force the cam 35outwardly first, further jamming occurs, because the sum of the lengthsof the straight lines 92 and 93 is greater than the length of the line91, and a wedging or toggle-like locking action takes place applyingpressure on the pins 36' and 76, as explained above.

The improved clamp structures of the present invention are particularlyadapted for lifting hard steel plates, such as heat-treated armorplates, alloy plates or stainless steel plates, which have smooth hardsurfaces, and which are difficult if not impossible to penetrate by thegripping cam of ordinary types of clamps, especially at the start of alifting operation. If the gripping cam of the clamp does not penetratethe plate somewhat, it is impossible to get the cam to function, sinceuntil the plate is gripped and held substantially, no load is applied onthe shackle. However, with the improved constructions ofthe presentinvention, a hand operated means is provided which acts in series withthe gripping cam of the grip. The penetrating clamp and the liftingshackle to effect a positive safe force must be great enough at thestart of the lifting operation and upon a small contacting surface inorder to cause penetration. With the hand operated screw and auxiliarycam arrangements of the present invention operating in conjunction withthe lever-actuated gripping cam to cause pressure at a single point ofcontacts, penetration sufficient to prevent slipping on thin hardsurfaces can be accomplished. If the hand screw, for example, and thecam were not operated in series with each other, but separately engagedthe plates side by side, the forces would be dissipated over the twopoints of contact and penetration would not be effectively accomplished.The engagement of a plate at two separate points by a clamp is notpractical or ef fective, because swiveling or swinging of the plate onone gripping point will hold, but such swinging or swivcling on twogripping points breaks them loose alternately.

A clamp constructed according to the invention is much safer thanpreviously known clamps, and in addition to having a greater initialgripping pressure, it is also locked in closed position on the plate andcan only be released by a hand operation, such as using a wrench tounscrew the Allen screw for one or two turns. In the construction shownin Figs. 1 to 4, there is no possibility of the wedged semi-nut beingreleased prior to using the Allen wrench, since it is locked in place bythe screw and is wholly independent of a spring to hold it in place. Thegripping cam cannot come olf the plate in normal usage until thepressure of the Allen screw is released. This is not true of ordinaryscrew clamps without a cam or of clamps provided with gripping camswithout a cooperating screw operating directly in series with the cam. Ascrewor C clamp when lifted with a crane can always be pulled off of anyplate when the plate is sufficiently heavy or loaded down. There isusually no permanent distortion of the clamp because the frame springsout under load and springs back when it is released, regardless of howstrong the frame is. My clamp on hard steel plates may be given theinitial pressure by the locking screw or locking cam to startpenetration of plate surface and the cam follows up (from the furtherforce brought about by the pulling of the shackle 52) the loosening thatmay be caused by any stretching of the frame or looseness of the lockcaused by further penetration of the cam. This pyramids up and asfurther pulling is exerted you get proportionately more gripping.Regardless of the stretching of the body of the clamp the grippingpressure keeps on mounting as the cams follow up and never release (as aC clamp does not follow up) until the clamp is destroyed or the plate ispulled apart. Ordinary clamps with gripping cams can be released onsmooth hard plates like polished armor plate or some stainless steels.The cam will not get penetration for the initial gripping which isnecessary to activate the force of the crane hook on the liftingshackle. Unless the cam has a penetrating force suflicient to preventslipping. at the very start of lifting, it will come off the plate.Clamps made according to the invention are provided with means forforcing some of the teeth of the gripping cam into the surface of theplate sufiiciently to hold until the lifting pressure is applied to thecam by the lifting shackle. The lifting shackle increases the pressureand wedging action of the gripping cam and may even deepen theindentations in the plate so that the grip is made even more secure.

The improved clamp structures of the present invention maintain acontinuous positive grip on the plate being lifted so that even if theplate were let down to rest on its edge, there would be no release ofthe clamp mechanism and the operator could pick up the plate again andknow that it would be perfectly safe. On the other hand, clamps providedonly with a gripping cam might release their grip if a plate is restedon its edge and allowed to tilt over. When the operator again tries tolift the plate, he may find that the clamp had released its grip andwould slip off the plate causing damage or injury.

In the use of lifting clamps of the type under consideration, theconfidence of the operator and plate handlers is very important. Thepositive locking grip of the improved clamp construction gives suchoperators and plate handlers positive confidence in their own safety,since the gripping cam is mechanically locked by a hand operatedmechanism and not dependent upon any spring locking or even on the pullof the lifting crane. Furthermore, the plate handlers know that, inorder to release the clamp, they must use a wrench or operate a handlever. This confidence is of great importance because for unknownreasons, plates have occasionally become disengaged and fallen whenbeing handled by previously known types of clamps.

The improved clamp of the present invention may be used for handlingother heavy objects than simply plates, and a single clamp, for example,can be effectively used to replace two of the previously known cam typeclamps used by millwrights to grab on the bottom of I beams. Where twoclamps are thus used, they are placed respectively on opposite sides.This operation can be very effectively accomplished with a single clampof the type disclosed herein and with a much greater degree of safety.Furthermore, a clamp constructed according to the present invention willsafely grab and lift tapered plates or articles having tapered edgeswhere they could not possibly be handled by prior clamps with grippingcams. The clamp of the present invention can be effectively used forlifting assembled parts where the edges are not horizontal, whereasprior known clamps would readily slip from such edges.

What I claim is:

l. in a lifting clamp of the type including a body having spaced opposeddepending portions providing a downwardly extending slot when the clampis in vertical lifting position said slot being adapted for receiving aplate or other article to be lifted, a gripping means on one of saiddepending portions facing the other depending portion across said slot,a gripping cam having a serrated arcuate article-gripping surface andpivoted in said other depending portion adjacent said slot with itsgripping surface facing toward the gripping means, and a leveragemechanism pivoted in the upper portion of said body and connected tosaid gripping cam for applying a gripping pressure to the gripping camwhen a lifting pull is applied to the free end of said leveragemechanism the improvement in which the gripping cam opposite itsgripping surface includes a convex arcuate cam surface having apredetermined pitch, a rotatable locking cam carried by the body of theclamp and located adjacent to the areuate cam surface of the grippingcam, and means for manually rotating the locking carn to apply pressureon the gripping cam and for releasing the locking cam, said locking camhaving a convex cam surface contacting the convex cam surface of thegripping cam and acting to hold the locking cam in its rotated lockingposition, the locking cam surface acting on the convex cam surface ofthe gripping cam to rock the gripping cam on its pivot and to force itsteeth into the surface of the article to be lifted, thereby providing apositive grip on the article to be lifted and preventing slipping of theclamp in the lifting of hard surfaced articles.

2. A lifting clamp as claimed in claim 1, including spring means carriedby the body of the clamp and acting on the locking cam for rotating andbiasing the locking cam into locking engagement with the gripping cam.

3. A lifting clamp as claimed in claim 1, in which.

the means for rotating the locking cam include a pivot pin on which thelocking cam is fixed, said pivot pin extending into the body of theclamp, a handle fixed to said pivot pin outside of the body of the clampfor rotating the pivot pin and locking cam, and means for latching thehandle in a position in which the locking cam is in its raised positionwith respect to the gripping cam.

4. A lifting clamp as claimed in claim 1 in which the locking cam ispivoted in the body of the clamp in alignment with the gripping earn,the cam suface of the locking cam being involuted and engaging the camsurface of the gripping cam at a point on that side of a straight lineextending through the axes of rotation of the cams which lies in thedirection of movement of the cam surface of the locking cam when thelocking cam is rotated in a direction to force its cam surface intoengagement with the gripping cam whereby the sum of the distances fromthe axes of rotation of the cams to the point of contact of the cams isgreater than the straight line distance between said axes of rotation.

5. A lifting clamp as defined in claim 4 in which the locking clamp ispivoted in the body of the clamp above the gripping cam.

6. In a lifting clamp of the type including a body having spaced opposeddepending portions providing a downwardly extending slot when the clampis in vertical lifting position said slot being adapted for receiving aplate or other article to be lifted, a gripping means on one of saiddepending portions facing the other depending portion across said slot,a gripping cam having a serrated arcuate article-gripping surface andpivoted in said other depending portion with its gripping surface facingtoward the gripping means, and a leverage mechanism pivoted in the upperportion of said body and connected to said gripping cam for applying agripping pressure to the gripping cam when a lifting pull is applied tosaid lifting mechanism, the improvement in which the gripping camincludes an involuted cam surface opposite its gripping surface having apredetermined pitch, an actuating cam pivoted to the body of the clampin alignment with and adjacent to the gripping cam for engaging androtating the gripping cam on its pivot and cooperating with the leveragemechanism to force the teeth of the gripping cam into firm engagementwith the surface of the article to be lifted, thereby providing apositive grip on the article to be lifted and preventing slipping of theclamp in the lifting of hard surfaced articles, said actuating camhaving an involuted cam surface of less pitch than that of the grippingcam, and hand operable means for rotating the actuating cam to force itsinvoluted cam surface against the involuted cam surface of the grippingcam.

7. In a lifting clamp of the type including a body having spaced opposeddepending portions providing a downwardly extending slot when the clampis in vertical lifting position said slot being adapted for receiving aplate or other article to be lifted, a gripping means on one of saiddepending portions facing the other depending portion across said slot,a gripping cam having a serrated arcuate article-gripping surface andpivoted in said other depending portion adjacent said slot with itsgripping surface facing toward the gripping means, and a leveragemechanism pivoted in the upper portion of said body and connected tosaid gripping cam for applying a gripping pressure to the gripping camwhen a lifting pull is applied to said leverage mechanism, theimprovement in which the gripping cam opposite its gripping surfaceincludes an involuted arcuate cam surface having a predetermined pitch,and a rotatable locking cam pivoted in the body of the clamp and locatedadjacent to the involuted arcuate cam surface of the gripping cam, andmanually-operable means for rotating the locking cam to apply pressureon the gripping cam, said locking cam having an involuted cam surfacecontacting the involuted cam surface of the gripping cam the pitch ofwhich is about one-half that of the involuted cam surface of thegripping cam, whereby the locking cam is held in its rotated lockingposition, the locking cam when rotated in the locking direction actingon the gripping cam to rock the gripping cam on its pivot end to forceits teeth into the surface of the article to be lifted, therebyproviding a positive grip on the article to be lifted and preventingslipping of the clamp in the lifting of hard surfaced articles.

References Cited in the file of this patent UNITED STATES PATENTS654,931 Aiken July 31, 1900 702,334 Schulte et a1. June 10, 19021,036,626 House Aug. 27, 1912 1,580,083 Boadle Apr. 6, 1926 2,370,411Monaco Feb. 27, 1945 2,393,101 Gardner Jan. 15, 1946 2,446,610 RenfroeAug. 10, 1948 2,497,293 Cade Feb. 14, 1950 2,542,289 Robbins Feb. 20,1951 2,543,017 Hagan Feb. 27, 1951 2,549,918 Miller Apr. 24, 19512,676,838 Gardner Apr. 27, 1954 2,700,452 Regimbal Jan. 25, 1955

